In most communication systems, including high definition television (HDTV) modulation and filtering are essential signal processing functions. While low cost microprocessors have made digital implementations of the filtering and modulation techniques practical, the demands of communication systems designers have surpassed the capabilities of available digital technology. The present invention relates to a new analog device that compliments existing digital signal processing techniques, and more particularly, relates to an acoustic charge transport (ACT) device such as described in U.S. Pat. No. 4,633,285 which is hereby incorporated by reference.
The development of powerful, low cost and high speed microprocessors has made sophisticated digital filtering very popular. Frequently, digital filtering is more accurate, repeatable, compact and less expensive than analog versions of a given filtering technique. One example of the popularity of digital signal processing is that most of the high definition television (HDTV) systems that are currently proposed require a massive amount of digital processing in order to, e.g., rearrange picture elements and to perform bandwidth compression. The all digital HDTV system proposed by Zenith Corporation is estimated to require approximately 19.4 billion digital operations per second. This system is described in Zenith Spectrum Compatible HDTV System, Sept. 1, 1988, which is hereby incorporated by reference. It is estimated that such digital processing will remain at the outer edge of obtainability for inexpensive microprocessors, even with new development of new processors in the next few years. Such processing can be done with a number of separate processors; but, this would significantly increase the cost of digital filtering and would most likely make the resulting HDTV system too expensive to be commercially practical. In an HDTV system, the video input has a frequency range of 0-28.4 MHz. In order to satisfy the Nyquist criterion, this input signal should be sampled at two times the highest input frequency. In the proposed Zenith system, the sampling frequency is approximately 75.6 MHz. Consequently, the input filters must process samples (e.g., all 19.4 billion operations per second) in the time between the samples which arrive at a rate of 75.6 MHz. Such high processing speeds require expensive processors, and for example, expensive input analog to digital converters. For many applications, therefore, digital signal processing is simply not practical. This gives rise to a significant need for an inexpensive alternative to digital signal processing.